1. Field of the Invention
This invention relates generally to astable multivibrator circuits of the type useful as an angle modulator or demodulator, and more particularly to an astable multivibrator circuit in which the duty cycle and the duty ratio thereof are controlled.
2. Description of the Prior Art
In the prior art, an emitter-coupled astable multivibrator including transistors is employed as an oscillatory element which is useful as an angle modulator or angle demodulator, for example, a frequency modulator or a frequency demodulator.
In the emitter-coupled multivibrator, a capacitor is connected between the emitter electrodes of two transistors, and the base electrodes of the two transistors are connected to the collector electrodes thereof through emitter follower circuits, respectively. In operation, the two transistors conduct alternately to generate the oscillations of the circuit. The frequency of the circuit is determined by the time of the charging and discharging of the capacitor, and the oscillations may be frequency-modulated by varying the charging or discharging time of the capacitor as a function of an input voltage. The emitter electrodes of the two transistors are normally connected to a common constant current source. Although the current is independent of variations in voltage across the capacitor and is constant in that sense, the magnitude of this current is variable in accordance with an input signal. In this manner, the charging current of the capacitor can be varied in accordance with the input signal. As a result of this control of charging current during one state of conduction of the multivibrator, the duration of one complete cycle is variable and therefore the frequency of the multivibrator is variable.
When a modulating signal, a video signal, for example, is applied to the base electrodes of transistors used as the constant current source, the current through the constant current source is varied, and therefore a frequency-modulated multivibrator signal is produced.
In the video recording circuit of a video tape recorder (VTR), a signal clamping circuit clamps the synchronizing peaks or the black level of the video signal, and the clamped signal is then passed through a pre-emphasis circuit and a white level clipping circuit to the frequency modulator. When the video signal is applied to the frequency modulator, the synchronizing, or sync, signal clipping level of the video signal is set to correspond to a predetermined frequency f.sub.o of the frequency-modulated multivibrator. This frequency, referred to as the carrier frequency, is set, or controlled, by changing the clamping voltage level in the signal clamping circuit. However, the white clipping level in the white level clipping circuit is undesirably changed or influenced due to changes in the clamping level in the signal clamping circuit, and consequently the white clipping level must be re-adjusted. It may take much time and require great skill to adjust these levels in the level clamping and the white level clipping circuits alternately to achieve satisfactory operation.